N-Acetylation is the main route of metabolic inactivation of INH and several other drugs which are widely used in medical therapy. Hereditary factors are important modifiers of this process and investigators have attempted to account for them in terms of the physiologic and biochemical attributes of the individual. One of the chief objectives of our investigation on this aspect of drug metabolism is to improve our understanding of its human pharmacology and pharmacogenetics through correlative genetic and biochemical studies in man and animals. Our approach to obtain this information is to identify and characterize factors which modify the enzyme (N-acetyltransferases) in tissues which are responsible for drug acetylation and their activity. A recent finding in our laboratory of interest is the identification of a genetically determined relationship between the N-acetyltransferase of peripheral blood cells and liver through studies inbred strains of rabbits known drug acetylator genotypes. The observation is of therapeutic interest since it might afford a basis for identifying persons who are especially susceptible to unwanted effects of a drug from a small sample of blood and without exposing the individual to the drug.